The objectives of this study are to gain insight into the mechanisms of pathogenesis of Chlamydiae. Our hypothesis is that there are special phylogenetic structure(s) responsible for (1) attachment/entry of susceptible cells, (2) initiation and maintenance of intracellular survival in both professional and non-professional phagocytic cells, and (3) immediate cytotoxicity. The specific aim of this proposal is to determine if the virulence factor(s) responsible for these biological events are located specifically on the elementary body envelope. The basic methods employed will compare the interaction of isolated, purified, I125-labeled elementary body envelopes coated onto latex spheres, quantitated to the appropriate multiplicity of infection, versus whole reticulate bodies with macrophages and tracheal ring organ culture cells.